1. Field of Invention
The present invention relates generally to the field of linear motors. More specifically, the present invention is related to a block switch controller for a linear motor that minimizes block switching transients.
2. Discussion of Prior Art
There are many types of linear motor systems including synchronous linear motors, switched reluctance linear motors, permanent magnet linear motors, and linear induction motors (LIM). In each case, the linear motor can be visualized as a rotary motor that has been sliced axially and “unwrapped”. A set of stator coils are energized and a force is generated that propels the moving shuttle across the stator windings but in a linear path rather than in a circular path. FIG. 1 shows a block diagram of a typical linear induction motor system 100 with a series of stationary stator coils called motor blocks 102 and a moving shuttle 104 which is analogous to the rotor of a rotary induction motor. The shuttle is often composed of a layer of aluminum and a layer of steel or conductive bars embedded in a steel back plate. The voltages are selectively applied to the stator coils by block switch triacs 106, also referred to herein as motor block switches with the two terms being used interchangeably, the applied voltages generating currents in the conductors and ultimately generate electromagnetic force that propels the shuttle along the track. Force is generated on the shuttle only by the stator blocks that are energized below the shuttle. The stator blocks are therefore energized directly beneath the shuttle and one block ahead of the shuttle so that the force is relatively constant as the shuttle traverses the track. The switching of applied voltage along sequential motor blocks is termed block switching. Some linear motor systems use a set of discrete sensors 110 such as optical or hall-effect sensors in order to determine the position of the shuttle relative to the motor blocks so that the proper motor blocks can be energized. Typically, there would be one or two of these sensors per motor block since only a crude measurement is required.
For vector current control and for precise motion control, a finer measure of position is generated from a device such a linear encoder 112. These might typically sense position down to a fraction of an electrical cycle. Encoder position feedback 130, stator current feedback 132, voltage feedback 134, and in some cases, coarse block sensor feedback 136 are available to control system 120. A typical control system consists of a processing element 122 such a computer or digital signal processor engine in combination with sensor conditioning and power electronics inverter 124 that provide voltages and currents suitable to operate the linear motor, including an operating voltage 140 to be supplied to the motor blocks, and block switching control signals 142 that control the activation and deactivation of the motor block switches 106. While not explicitly illustrated in FIG. 1, a block switch controller is a subcomponent of the control system which is known in the art and is responsible for determining when the block switches should be activated and deactivated.
Switching inductive motor blocks in and out of the circuit as the shuttle travels along the track creates transient conditions that can affect motor performance. Two main areas of concern are transient saturation of the core as the new coil is engaged and transient load imbalance. These effects can result in transient current surges that lead to component damage and transient forces that upset smooth operation. The resultant effects on the system can also be dependent upon the degree of interaction with the control algorithm being used and the operating point of the linear motor.
Prior work on linear motor systems does not address the practical issues of linear motor block switching. Many previous applications with linear motors are of limited length and limited power and therefore can energize all coil segments without too much of a power loss penalty. Others employ a single moving coil design with a long fixed reaction plate such that no block switching is needed. These prior art systems require long wires carrying power that are attached to the moving shuttle and eventually wear down due to mechanical stress. Neither of these architectures is suitable for use in a high power, high acceleration application, such as an electromagnetic aircraft launch system (EMALS) application.
Whatever the precise merits, features, and advantages of the above cited references, none of them achieves or fulfills the purposes of the present invention.